Proposed federal and state regulations on controlling emissions from internal combustion engines generally call for reductions in nitrogen oxide (NO.sub.x) while keeping particulate emissions at or below current levels. Representatives of the diesel engine industry and regulatory agencies previously signed a Statement Of Principles which calls for NO.sub.x emissions of 2.5 grams per brake hose power-hour (g/bhp-hr) or less and particulate matter emissions of 0.10 g/bhp-hr or less by the year 2004. The U.S. Environmental Protection Agency (EPA) has proposed new regulations based on the Statement of Principles. The EPA issued a notice of proposed rule making entitled Control of Emissions of Air Pollution from Highway Heavy-duty Engines (61 F.R. 33421, Jun. 27, 1996) with proposed changes to 40 C.F.R. Part 86.
In the past significant progress has been achieved in reducing diesel engine emissions by various changes in engine design and fuel system design. Fuel improvements and exhaust after treatment techniques have also been used to meet the challenge of lower allowable levels of engine exhaust emissions. At the same time, customers are demanding greater fuel efficiency and extended engine life with less maintenance requirements. As a result, several difficult design tradeoffs must often be made to meet these sometimes conflicting goals. For example reducing NO.sub.x emission from a diesel engine by retarding injection timing may have a negative impact upon fuel economy. Also, design changes made to reduce particulate emissions may increase NO.sub.x emissions and vice versa. The task of maintaining good fuel economy is especially difficult with the need to control NO.sub.x and particulate emissions at the new, proposed relatively low levels in comparison with prior acceptable standards. A paper entitled Progress in Diesel Engine Emissions Control by Magdi K. Khair was presented at the ASME Energy-Sources Technology Conference and Exhibition during January 1992 in Houston and provided a summary of previous changes made to improve performance while reducing emissions from diesel engines.
Two technologies, when combined, hold promise in helping heavy-duty diesel engine designers meet the future EPA emission regulations. Exhaust gas recirculation (EGR), a technology used for some time in light-duty diesel engines, has been effective in reducing NO.sub.x emissions to levels approaching those proposed by the new regulation. Exhaust gas recirculation reduces NO.sub.x in diesel engines by diluting the oxygen induced with the fresh charge air as well as acting as a heat sink in the combustion process. A serious consequence of this approach is an increase in insoluble particulate matter (primarily soot).
Exhaust gas after treatment for diesel engines by filter traps have proven to be effective for some applications in dealing with insoluble particulate matter. Diesel exhaust after treatment has traditionally been characterized by high cost and low reliability. Recent developments in passively regenerated filter traps using fuel additive catalyzing agents have emerged as a lower cost alternative to conventional active regeneration filter trap systems.
Several diesel engine manufacturers have experimented with EGR and passive filter trap technology and successfully reduced NO.sub.x emission to approximately 2.0 g/bhp-hr or less while obtaining extremely low particulate matter emissions. Lab demonstrations have shown that these two technologies allowed several diesel engines to meet the EPA proposed model year 2004 heavy-duty diesel engine exhaust emissions standards.